Field of the Invention
The present invention relates to an optical receiver circuit enabling coherent detection in optical fiber communication in which high-capacity and high-speed data transmission is performed.
Description of the Related Art
In optical fiber communication, a technique enabling high-capacity and high-speed data transmission includes digital coherent communication. In the digital coherent communication, an optical receiver circuit for performing coherent detection is necessary. Toshikazu Hashimoto, Yohei Sakamaki, Hiroshi Yamazaki, Takayuki Mizuno, Takashi Goh, Yusuke Nasu, Kunisaku Hattori, Hiroshi Takahashi, “Dual polarization optical hybrid module using planar light wave circuit”, 2009 Society Conference of Institute of Electronics, Information and Communication Engineers, C-3-39 (hereinafter, referred to as Non-Patent Document 1) discloses a 90-degree hybrid circuit configured to include an intersection portion in which a waveguide for propagating signal light and a waveguide for propagating local-oscillator light intersect each other on an input waveguide side. The 90-degree hybrid circuit detects a light signal in a quadrature phase-shift keying (QPSK) format which has orthogonal phase components, in coherent detection.
In the digital coherent communication, QPSK signals are multiplexed into orthogonal polarization states, and thus an optical circuit performing polarization splitting is necessary. Non-Patent Document 1 discloses an optical circuit, performing polarization splitting, which is integrated on the same substrate as two 90-degree hybrid circuits to which two polarization states orthogonal to each other are respectively input.
There is a problem in that the quality of coherent detection can deteriorate due to the mixing of signal light and local-oscillator light in the intersection portion of the waveguide. Additionally, since polarization rotates in the intersection portion, a detection error can occur due to inter-polarization crosstalk, and the amplitude of a received signal in coherent detection can be reduced due to the generation of an incoherent component, which can result in a problem of deterioration in noise tolerance.
As described above, in the related art, it can be necessary to separate an optical hybrid circuit for detecting quadrature phase components into that for each phase component in order to perform high-quality coherent light detection by using a single-mode external local-oscillator light source having a narrow line width. In this case, a polarization splitting circuit is also configured as an optical circuit separated from the optical hybrid circuit, and thus it is difficult to reduce the size of an optical receiver circuit. As a result, it is difficult to reduce the dimension of a light transmission device and to reduce costs.
An object of the invention for solving such problems is to realize an optical receiver circuit which is integrated on the same substrate as an optical hybrid circuit (further, a polarization splitting circuit and an optical hybrid circuit) without using an intersection waveguide, and to provide a small-sized optical receiver circuit enabling high-quality coherent light detection.